The term input/output module is used hereinbelow to refer either to the individual module having respectively a connection facility for external peripherals such as e.g. switches, limit monitors, electrical, hydraulic or pneumatic units and such like, or, particularly in the case of so-called compact programmable controllers, i.e. devices in which the control and processing functionality and the facilities for connecting peripherals are combined in one device, to the entire programmable controller.
Programmable controllers of the type cited in the introduction, particularly in an embodiment as modular programmable controllers, are generally known e.g. in the form of programmable controllers which are supplied by the applicant under the SIMATIC brand. From these programmable controllers, input/output modules fashioned as individual modules, in particular input modules or combined input/output modules, as a concrete example of an input/output module, are also known.
The invention relates specifically to an input/output module comprising at least one signaling contact functioning as a digital input. Such an input/output module constitutes accordingly an input/output module with digital input functionality. Such modules are frequently fashioned with a plurality, e.g. 16 or 32, of digital inputs and are designated digital input modules accordingly. The digital input module is thus a special form of input/output module with exclusively digital input functionality. Besides these, mixed forms, that is e.g. digital input/output modules or digital input and analog input modules, can also have at least one signaling contact functioning as a digital input. The peripheral devices covered by the invention are correspondingly also collectively referred to in short below by the term digital input module or input module or module, the key criterion being invariably the at least one signaling contact functioning as a digital input.
The input modules known in the prior art are based on the principle that a current flows into the module via a digital input, that is, the input channel linked to the signaling contact, when a connected sensor is activated. In the simplest case, the sensor is recognized as activated when a current is flowing. Correspondingly, the sensor is recognized as deactivated when there is no current flowing. However, even where a sensor is deactivated, a current can flow into the module so the status of the sensor is normally recognized from the magnitude of a voltage, preferably from the exceeding of certain predefined or predefinable threshold values for the voltage magnitude which is required for driving a current via the signaling contact.
This standard principle is not quite optimal insofar as a power loss arises inside the module as a result of the input current. In standards specified under IEC 61131-2 for voltage and current values at digital inputs (type 1: high signal in the voltage range 15V . . . 30V, input current at high signal: min. 2 mA; type 2: high signal in the voltage range 11V . . . 30V, input current at high signal: min. 6 mA), the power loss in an input channel of the above-mentioned second type, which is also suitable for connecting so-called BEROs (contactlessly operating sensors), amounts to at least 30V×6 mA=180 mW. The power loss also varies depending on the electrical connection of the input channel. In the case of an input channel realized by means of a passive circuit, a power loss of 30V×16.4 mA=492 mW occurs. The preceding numerical example (power loss: 180 mW) relates to an “active circuit” with a so-called current sink.
The power loss shown above by way of example arises, moreover, in each input channel, i.e. in the case of a module comprising 32 input channels, the power loss amounts to a total of 32×180 mW=5.76 W (32×492 mW=15.74 W). As regards a desirable reduction in the power loss, however, it is not possible to reduce the magnitude of the input currents, since it is not otherwise possible to comply with the requirements under IEC 61131-2.
Furthermore, it must be taken into account that efforts are being made to develop 64-channel modules. By extrapolation, power losses of 64×180 mW=11.5 W and 64×492 mW=31.5 W will arise respectively for the implementation of all inputs with an active and passive circuit.
The power loss must be dissipated from the module and, as a result, necessitates a certain size for the module. Since the sizes theoretically required run contrary to the trend toward miniaturization, a derating of modules is frequently carried out, i.e. the user has to take care to ensure that only a certain predetermined number of input channels is activated simultaneously (permanently). In unfavorable cases, this may mean that e.g. only half of the inputs available can actually be used.